Respiratory failure with CO2 retention is commonly associated with diseases which either increase the work of breathing or compromise respiratory muscle contractility. In either case the load per unit of intact respiratory muscle is increased. The objectives of this project are to determine the extent of respiratory muscle dysfunction in diseases which predispose to respiratory failure, to evaluate the extent to which respiratory muscle dysfunction contributes to the development of hypercapnia, and to identify among factors which impair respiratory muscle performance those which are potentially reversible. To accomplish these goals three general protocols will be employed. The first involves prospective observation of respiratory muscle function in patients with obstructive and restrictive thoracic and pulmonary disorders, and in patients with respiratory muscle weakness secondary to neuromuscular disease. Respiratory muscle strength and endurance to short intensive loads will be tested using conventional methods. In addition, an attempt will be made to test respiratory muscle endurance for longer periods at loads which approximate those encountered in disease. The mechanics of the respiratory system will be investigated in order to assess the balance between respiratory muscle endurance capacity and the force necessary to maintain adequate levels of ventilation, both at rest and during walking at slow to moderate speeds. The second protocol will assess respiratory muscle function and respiratory system mechanics in patients undergoing intensive treatment for acute respiratory failure. Measurement of diaphragmatic and other respiratory muscle electrical activity will be employed to determine the extent of uncoordinated respiratory muscle contraction. The third protocol is designed to determine the effects on respiratory muscle strength and endurance of nutrition and generalized muscular development in patients who are free from primary pulmonary, thoracic or respiratory muscle disorders.